Substantial progress has been achieved in recent times in determining the parameters responsible for the activity of individual genes. Less is known concerning the long-standing problem of coordinate gene regulation. We are studying the phenomenon of dosage compensation in Drosophila in order to understand certain novel and essential aspects of eukaryotic gene regulation. Dosage compensation refers to the equalization of most X-linked gene products in males with one X chromosome and one dose of X-linked genes and in females with two X's and two doses of such genes. Since the x chromosome represents a very large proportion of the enchromatic karyotype of Drosophila, dosage compensation is unique among regulatory mechanisms in that it modulates the transcriptional activity of a very large number of genes. Another unique feature of dosage compensation is that it is regulated by well-characterized genes which also control the initial steps in the pathway leading to sexual differentiation. Our experimental goal is to understand the underlying regulatory mechanism(s) of dosage compensation at the molecular level. To this end we propose to achieve the following specific aims: (1) We will identify the X chromosome sequences that enhance the rate of transcription of X-linked genes in males; we will first concentrate on a house-keeping gene (Pgd+) and will return later to the study of Zw+ for comparative purposes. (2) We will identify the regulatory factors responsible for transcriptional enhancement in males. We will use a DNA/protein binding screen and DNA/protein binding assays with X-linked regulatory sequences to identify such regulatory factors. We will also complete the cloning of the regulatory genes msl-l+ and msl-2+ and clone msl-3+. (3) we will search for and characterize new regulatory genes.